555 alternate dual led string fader (breathing effect)

Thread Starter

digital_citizen

Joined Jun 22, 2015
15
Hi I'm having some trouble in building a circuit based on one or max two 555 timers and at this point I don't know if it's even possible.
Basically I want to achieve a breating effect (slow on, slow off) of two strings of led (150mA max, so perfectly fine for 555 output) using a 555 timer and so far I've came across this schematic:
555-7-2.png
First of all this circuit doesn't have enough power, and second the fading effect is only during on time but not during off time (it does turn off abruptly). The supply voltage is 5V so I've added two transistors at pin3 (after the capacitor) but it doesn't work: I have an ugly effect.

So my question is: It's possible to achieve an alternating breathing effect with 555 or not?
 

dl324

Joined Mar 30, 2015
13,141
The general consensus is that it's better to use PWM (Pulse Width Modulation) to vary the brightness of LEDs.

Here's a waveform example:
clipimage.jpg

A comparator is given as its inputs the triangle wave and the level voltage. Whenever the triangle voltage is lower than "level", the output will be HIGH. The frequency is determined by the square wave.

So you build something like a 1kHz square/triangle wave generator using a couple opamps. To adjust LED brightness, you change the "level" voltage. Higher means the LED is on more, lower means the LED is on less.

EDIT: You can change the polarity of what I described by swapping the comparator inputs or driving the LEDs high side instead of low side.

Schematic for the circuit is:
clipimage.jpg
Originally discussed here.
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
1,814
EDIT:

Very sorry, but I had to withdraw the circuit...once again I missed the fact that the supply voltage was 5volts and not 12, the circuit I posted won't work very well at 5 volts.
 
Last edited:

Thread Starter

digital_citizen

Joined Jun 22, 2015
15
Here is a linear circuit I designed for a project I did a little while back, It will dissipate more power than PWM, but at 150 mA it's not really that much.

One LED brightens while the other dims. Remove the PNP section for a single LED.

View attachment 221798

Adjust as needed. (timing resistors and caps...etc)

The 1k on the output pin is not needed, but the feedback to the control voltage is. (also adjust as needed)
OK that's the closest thing I've been looking for: it's linear, uses one 555...but if it's not much is possible to omit the opamps, since I'm in a relatively hurry and don't have the time to order parts?
 

AnalogKid

Joined Aug 1, 2013
9,361
The post #1 schematic shows something that I guess is a yellow LED. Why is it there?

Usually, an LED breathing circuit has one transistor on the output of the 555. Here is a grab off the innergoogle.

UPDATE: Removed 2nd schematic. To have a complimentary LED, add a PNP transistor with the 2nd LED going to Vcc through its own current limiting resistor. Both bases connect to R1.

ak
1604925675400.png
 
Last edited:

Thread Starter

digital_citizen

Joined Jun 22, 2015
15
I want two alternating strings of LEDS that are fading on and off one after another (while one turns slowly on, the other turns slowly off, and so on). all the schematics you've posted are for just one led
 

ElectricSpidey

Joined Dec 2, 2017
1,814
Perhaps you could modify the circuits in post #8 by driving a PNP with the NPN, and put the second string on that.

I don't know, I don't have time to test.

Also, if you tie back the output to the control voltage you can change the crossover by modifying the threshold levels.
 

djsfantasi

Joined Apr 11, 2010
7,842
I want two alternating strings of LEDS that are fading on and off one after another (while one turns slowly on, the other turns slowly off, and so on). all the schematics you've posted are for just one led
I understood that’s what you wanted. But baby steps. You need to light ONE LED first, then TWO with the desired pattern. This circuit, with drivers, should light your LED strips as well as single LEDs.

You have been in competent hands, so I haven’t chimed in. But ElectricSpidey has a good idea. Basically, you need to invert one signal to get the second signal. Likely the LEDs have one side common, so you’ll need to switch either the high or low side. So instead of an NPN switching a PNP, consider one (NPN?) transistor switching one strip and a second transistor (NPN, as an inverter) that subsequently switched the second LED strip.
 

ElectricSpidey

Joined Dec 2, 2017
1,814
I have tried this kind of configuration where the bases are connected together, it works but...there can be some undesirable results at the transition points.
 

Thread Starter

digital_citizen

Joined Jun 22, 2015
15
I have tried this kind of configuration where the bases are connected together, it works but...there can be some undesirable results at the transition points.
Yes, I've tryed also some similar arrangement and it doesn't work properly (one led string doesn't turn off). Although I have to try this particular schematic that AnalogKid have posted and I will let you know very soon.

I understood that’s what you wanted. But baby steps. You need to light ONE LED first, then TWO with the desired pattern. This circuit, with drivers, should light your LED strips as well as single LEDs.

You have been in competent hands, so I haven’t chimed in. But ElectricSpidey has a good idea. Basically, you need to invert one signal to get the second signal. Likely the LEDs have one side common, so you’ll need to switch either the high or low side. So instead of an NPN switching a PNP, consider one (NPN?) transistor switching one strip and a second transistor (NPN, as an inverter) that subsequently switched the second LED strip.
Do you mean what exactly said AnalogKid?
 

djsfantasi

Joined Apr 11, 2010
7,842
Yes, I've tryed also some similar arrangement and it doesn't work properly (one led string doesn't turn off). Although I have to try this particular schematic that AnalogKid have posted and I will let you know very soon.



Do you mean what exactly said AnalogKid?
No, not at all.

Regardless of pull up/pull down/ base resistors, connect as follows (I don’t have tools at hand to draw a proper schematic)...
  1. 555 to base of first NPN transistor (Q1)
  2. First transistor emitter to ground; collector to led strip AND to second NPN transistor base
  3. Second transistor emitter to ground; collector to second led strip
 

ElectricSpidey

Joined Dec 2, 2017
1,814
" Yes, I've tryed also some similar arrangement and it doesn't work properly (one led string doesn't turn off) "

Were you using two different color strips? For example if one is red and one is blue a diode in series with the red can help.

There can also be problems because the output of a bjt 555 is not symmetrical.

Also, when the bases of NPNs and PNPs are directly connected they can drive each other, and this must be considered in the design.
 

AnalogKid

Joined Aug 1, 2013
9,361
First pass at a *concept* schematic. Note that no 1% parts are required; they are what's in my design libraries.

The voltage at node R1-C1 varies between 0.33Vcc and 0.67Vcc. The difference between max and min brightness depends on the LED forward voltage Vf, the transistor base-emitter voltage Vbe, and Vcc. The lower Vcc is, the greater the difference between max and min brightness.

A dual-opamp square/triangle wave circuit can give a much greater (and adjustable) difference between max and min brightnesses.

ak
LED-Breathe-1-c.gif
 
Last edited:
Top